As far as CPU resources are concerned, which anti-virus software running in the background for real time protection is the best fit for Pentium III class machines (@650 MHZ) with 512MB of RAM? Sorry, if this is a duplicate question.
Im not really sure about which uses the least cpu power. But most AV programs dont use very much cpu power when they are just running in the backround, unless your actually doing a scan. When scanning on an older computer like yours, things are going to slow down considerably regardless what AV program your using, so your gonna wanna schedule scans for a time when ur not going to be using the computer, while ur sleeping for example. As for the real-time protection effecting performance of your PC you really wanna take a look at how many processes and how much memory the program is using especially if you dont have alot of memory to spare. I have an older notebook I still use which is a Duron 1.2ghz/768mb so I know what its like to have limited resources. I have used a few differant virus programs on this pc, I agree with the above reccomendation for AVG free if you dont wanna spend any money, but there are much better prgrams out there. I have settled on NOD32 AV it has worked well for me and takes up very little resources. Its among the best virus protection you can get and uses very little system resources, only 20mb.
You cant really say that free programs work better then paid ones. There are good and bad in both categories. One thing to consider is that the free software doesnt usually offer any sort of Help/Support. To a beginner being able to speak to someone if you have a problem may be worth the $30 or so it cost for the software. I prefer NOD32 its very effective and uses very little memory, its well worth the $30 i paid.
What I meant was that Avast!, NOD32 and AVG have far fewer complaints than the others that I mentioned. True all 3 have paid versions but my post was not targeted at them rather it was targeted at Kaspersky, Symatic and McAfee.
Maybe I should have phrased my post differently. My bad.
After using avast for a while I switched to the paid version (needed command line feature) and don't regret it one bit.